A carbon nanotube is a tubular substance having a diameter of about 0.4 nm to several tens of nanometers obtained by rolling one layer of graphene sheet (a layer made of a six-membered carbon ring) into a cylinder. The carbon nanotubes are attracting people's attention as an excellent nanomaterial having thermal and chemical stability, mechanical strength, electron conductivity, thermal conductivity, and spectral characteristics that extend up to a near infrared region.
Among the carbon nanotubes (which may hereafter be referred to as “CNT”), there are a single-walled carbon nanotube (SWCNT) made of only one layer of the graphene sheet, a double-walled carbon nanotube (DWCNT) made of two layers of the graphene sheets, and a multi-walled carbon nanotube (MWCNT) made of two or more layers of the graphene sheets.
As an application example of carbon nanotubes, a carbon nanotube composite electrode (CNT composite electrode) in which the carbon nanotubes are fixed onto an electrode substrate is being developed.
For example, Japanese Patent Document 1 (JP-2008-64724 A) discloses a CNT composite electrode provided with CNTs that are grown from a metal catalyst fixed onto an electrode substrate.
By this method, the CNTs are formed directly on the electrode substrate, so that the electron movement between the CNTs and the electrode is easy, and the CNT composite electrode can be used as a highly sensitive sensor. On the other hand, the bonding force between the CNTs and the electrode is insufficient, so that the CNTs are liable to drop off. In particular, when it is necessary that the CNT composite electrode is immersed in water for application as a sensor or the like, there is a problem in that the CNTs are easily desorbed.
Japanese Patent Document 2 (JP-2005-332612 A) discloses a method for manufacturing a CNT composite electrode in which CNTs are immobilized by depositing on an electrode substrate a mixture of fine particles containing a major component of the electrode substrate and CNTs and heating the resultant in oxygen-free atmosphere to turn the fine particles into a coating film.
In this CNT composite electrode, the CNTs are fixed while being interposed between the fine particles containing the major component of the electrode substrate, so that the CNTs are firmly fixed onto the electrode. However, at the time of fixing, the CNT wall surface is damaged by the fine particles containing the major component of the electrode substrate, so that the intrinsic characteristics of the CNTs cannot be sufficiently obtained. Also, this method has a disadvantage in that the amount of CNTs that can be fixed per unit electrode area cannot be made so large.